Long-sought carbonate minerals found on Mars

Carbonate minerals, which form in the presence of water and have previously been found only in trace amounts on Mars, have been spotted in outcrops of rock on the Red Planet, new observations reveal.

Since acidic conditions can prevent carbonates from forming, the discovery suggests that the minerals were created in neutral-pH water that might have provided a cosy habitat for life.

On Earth, oceans absorb carbon dioxide from the atmosphere and deposit it as carbonate rock. But although some evidence points to past water oceans on Mars, only small amounts of carbonates have been found there - in Martian meteorites that landed on Earth and in atmospheric dust and bright soils on the planet.

In 2004, iron-rich minerals and sulphate salts observed by NASA's Opportunity rover suggested an explanation: any oceans were too acidic for carbonates to form.

Indeed, a study in 2006 suggested that Mars may have gone through an acidic phase, triggered by active volcanism, after an early period in which it had a denser atmosphere and large bodies of neutral-pH water on its surface.

Now, a study led by Bethany Ehlmann of Brown University in Providence, Rhode Island, reports the discovery of magnesium carbonate rocks in a region of Mars called Nili Fossae, as well as smaller carbonate deposits in a couple of other sites (see map). The subtle spectral signature of carbonates was identified by the high-resolution CRISM spectrometer on NASA's Mars Reconnaissance Orbiter.

'Fairly spectacular'

The carbonates in Nili Fossae are found in rocky outcrops that each cover no more than 10 square kilometres across. The minerals are thought to have formed when liquid water - either from subsurface groundwater or from shallow, ephemeral lakes on the surface - came into contact with the mineral olivine.

"In some places, like the Opportunity landing site, Terra Meridiani, it's clear that acid waters dominated the region," Ehlmann told New Scientist. "Because we hadn't seen carbonate bearing rocks before, we extrapolated this could be true over much of the planet."

"But at least in pockets, like where we see the carbonate, it suggests waters were mostly neutral to alkaline," she says. Such waters represent "a different sort of aqueous environment - potentially a habitat for micro-organisms - on ancient Mars".

Mark Bullock of the Southwest Research Institute in Boulder, Colorado, is impressed with the results. "This is a fairly spectacular paper," he told New Scientist. "[These rocks] aren't the massive layered limestones we see on Earth, but they are extensive enough to suggest that neutral pH waters in that location were necessary for their formation."

Complex history

What's more, the carbonates appear in rocks that seem to be younger - between 3.5 billion and 3.9 billion years old - than those in which the sulphates seen by Opportunity formed. "What this means is that Mars didn't just go from wet and pH neutral to drier and more acidic as it got colder," Bullock says.

"There must have been regional climate and chemical conditions that varied in time and space," he says, adding that this would occur as the tilt of Mars's spin axis changed over tens of thousands of years, a wobble caused by the lack of a massive moon to stabilise the planet. "I think it just goes to show that Mars has probably had a pretty wild climate history, what with the pole swinging all over the place."

Joshua Bandfield of the University of Washington in Seattle agrees: "The new results add another interesting piece of information which supports a more complex model for the history of the planet."

Hidden carbonates?

Just how common are carbonates on Mars? "I think that between the dust and the meteorites, it is apparent that carbonates are likely to be widespread, but not at high concentrations," says Bandfield, who adds that it is unclear how concentrated the carbonates in Nili Fossae are.

"We're going to keep looking as more CRISM data comes down," says Ehlmann. "[But] I think it's unlikely that we will find another big, regional unit like we see in Nili Fossae."

Bullock agrees: "I think there must have been limited times and limited places on Mars where carbonate rocks could have formed, perhaps at the bottom of a lake that persisted for a while."

Target site

Bullock adds that Nili Fossae "seems like a very compelling new candidate landing site for the Mars Science Laboratory."

In fact, Nili Fossae was initially considered as a possible landing site for the giant rover because it contains clays that must have formed in water. Intriguingly, it also might be a source of methane gas detected in the Martian atmosphere that might have a biological origin.

Technical glitches recently forced NASA to postpone MSL's launch from 2009 to 2011. That means researchers will reconsider the question of where to land the rover in about a year, says MSL project scientist John Grotzinger of Caltech.

"However, the reason the Nili landing site was not advanced was because it is too high in altitude - too risky for landing," he told New Scientist.

The massive lander needs to travel through as much of Mars's thin atmosphere as possible to slow down enough to land safely, ruling out sites, such as Nili Fossae, that lie at high altitudes. "Unfortunately, this constraint will also apply in 2011," says Grotzinger.

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In this close-up view of a region of ancient, eroded terrain in Nili Fossae, carbonates (bright green) formed a few billion years ago when water interacted with the mineral olivine (yellow). Clays (light blue), which also formed in water, lie beneath the olivine and carbonate rocks (Image: NASA/JPL/JHUAPL/U of Arizona/Brown U)